1. Field of the Invention
The present invention relates to a lubricating oil composition for traction drive and, more particularly, to a lubricating oil composition which has a high coefficient of traction, shows a limited change in the coefficient of traction at low temperatures, excels in wear resistance, load carrying capacity, thermal and oxidation stability and rust preventing property, and can effectively be used as the lubricating oil for power transmission systems including a traction drive mechanism.
2. Statement of the Prior Art
In recent years, traction drives (friction driving device utilizing rolling contact) have been applied as the continuously variable transmission for automotive and industrial purposes. The fluids used for such traction drives are required to have a high coefficient of traction and excel in power transmission efficiency.
A number of proposals have been made of the traction drive fluids meeting such demands (for instance, Japanese patent publication Nos. 46-338, 46-339, 47-35763 and 58-27838). All these proposals have their objects to improve the coefficient of traction.
As a result of testing the performance of such fluids for traction drive with actual machines, however, it has been found that they are poor in power transmission ability at low temperatures, and are thus practically unsatisfactory. In power transmission systems exposed to low to high temperature environements, the coefficient of traction of these fluids changes depending upon temperature and, in particular, drops remarkably at low temperatures. Thus, problems arise in connection with practical performance in winter and cold districts.